The invention is based on an apparatus for determining the flow rate, or mass, of a flowing medium as defined hereinafter. From U.S. Pat. No. 4,399,697, an apparatus is known that has two resistors embodied as films on a substrate; the first film acts as a heating resistor and is secured directly on the substrate and the second film is embodied as a temperature-dependent measuring resistor and rests on the first film, with an electrically insulating film located between them. The insulating film separating the heating resistor and the measuring resistor is so thin that good heat transmission from the heating resistor to the measuring resistor is possible. Variations in the flow rate lead to a variation in the transmission of heat at the surface of the measuring resistor and the substrate and, since the measuring resistor is temperature-dependent, to a variation in the electrical resistance of the measuring resistor. The result is imbalancing of a measuring circuit, which is compensated for by varying the heating current at the heating resistor. Serving as the standard for the mass or flow rate of the flowing medium is the electrical power supplied to the heating resistor. An additional temperature-dependent resistor, which is disposed on an additional substrate, serves to balance the temperature of the medium.
A disadvantageous feature of this known apparatus is that not only the flow of heat convectively given up to the medium to be measured, but also the flow of heat that is transferred by thermal conduction from the substrate to the substrate holder, is detected. Since the substrate has a large thermal capacity in comparison with the resistors, unwanted heat transmission of this kind slows the starting up process that lasts until the required operating temperature of the measuring apparatus is reached and also makes for a slow response to changes in the flow rate of the medium. Hence the geometrical arrangement of the measuring resistor in the known apparatus cannot be considered optimal, because lateral outflows of heat from the substrate have a deleterious effect on the accuracy of the measurement result. The measuring resistor therefore has a low speed of response to changes in the flow rate.
The known apparatus also has the disadvantage of a strong tendency to becoming soiled, since especially when it is used in the intake tube of an internal combustion engine, particles of dirt from the intake tube atmosphere become deposited on the leading edge of the measuring resistor and over long-term operation negatively affect the measurement result.